First-principles calculations are performed to study the electronic and magnetic properties of VX(2) monolayers (X = S, Se). Our results unveil that VX(2) monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine 2D monolayers. Furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain from -5% to 5% for VX(2) monolayers. It is proposed that the strain-dependent magnetic moment is related to the strong ionic-covalent bonds, while both the ferromagnetism and the variation in strength of magnetic coupling with strain arise from the combined effects of both through-bond and through-space interactions. These findings suggest a new route to facilitate the design of nanoelectronic devices for complementing graphene.

The geometric and electronic structures of graphene adsorption on MoS(2) monolayer have been studied by using density functional theory. It is found that graphene is bound to MoS(2) with an interlayer spacing of 3.32 Å and with a binding energy of -23 meV per C atom irrespective of adsorption arrangement, indicating a weak interaction between graphene and MoS(2). A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS(2)/graphene hybrid accompanied by a small band-gap (2 meV) opening due to the variation of on-site energy induced by MoS(2). These findings are useful complement to experimental studies of this new synthesize system and suggest a new route to facilitate the design of devices where both finite band-gap and high carrier mobility are needed.

Very recently, two-dimensional nanosheets of MoSe(2), MoTe(2) and WS(2) were successfully synthesized experimentally [Science, 2011, 331, 568]. In the present work, the electronic and magnetic properties of perfect, vacancy-doped, and nonmetal element (H, B, C, N, O, and F) adsorbed MoSe(2), MoTe(2) and WS(2) monolayers are systematically investigated by means of first-principles calculations to give a detailed understanding of these materials. It is found that: (1) MoSe(2), MoTe(2) and WS(2) exhibit surprising confinement-induced indirect-direct-gap crossover; (2) among all the neutral native vacancies of MoSe(2), MoTe(2) and WS(2) monolayers, only the Mo vacancy in MoSe(2) can induce spin-polarization and long-range antiferromagnetic coupling; (3) adsorption of nonmetal elements on the surface of MoSe(2), MoTe(2) and WS(2) nanosheets can induce a local magnetic moment; H-absorbed WS(2), MoSe(2), and MoTe(2) monolayers and F-adsorbed WS(2) and MoSe(2) monolayers show long-range antiferromagnetic coupling between local moments even when their distance is as long as ∼12 Å. These findings are a useful addition to the experimental studies of these new synthesized two-dimensional nanosheets, and suggest a new route to facilitate the design of spintronic devices for complementing graphene. Further experimental studies are expected to confirm the attractive predictions.

Recently, extensive experimental and theoretical studies on single layers of BN, GaN and graphene have stimulated enormous interest in exploring the properties of these sheets by decorating their surfaces. In the present work we discuss half-fluorinated single layers of BN, GaN and graphene, in the context of intercoupling between strain and magnetic property. First-principles calculations reveal that the energy difference between ferromagnetic and antiferromagnetic couplings increases significantly with strain increasing for half-fluorinated BN, GaN and graphene sheets. More surprisingly, the half-fluorinated BN and GaN sheets exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain, even giving rise to half-metal when the sheets are under compression of 6%. It is found that the magnetic coupling as well as the strain-dependent magnetic transition behavior arise from the combined effects of both through-bond and p-p direct interactions. Our work offers a new avenue to facilitate the design of controllable and tunable spin devices.

Very recently, the graphene@MoSe2 heterobilayers [G@MS HBLs] were successfully synthesized experimentally. In this work, the adhesion and electronic properties of the G@MS HBLs have been studied by using density functional theory. It is found that the graphene is weakly bound to the MoSe2 monolayer without any site selectivity. The bands of G@MS HBLs have characteristic graphene-like features with a small band gap (2 meV) opening at K. However, the gap value is significantly lower than k B T at room temperature and can even vanish at room temperature, which suggests that the unusual electronic properties of graphene in G@MS HBLs are comparable to those in the free-standing graphene. Thus, the MoSe2 monolayer might be considered as a promising substrate for potential graphene-based devices. The physical origin of the semiconducting property is related to the inhomogeneity of the onsite energy of C atoms induced by MoSe2. Our results provide a detailed understanding of the interfacial properties of G@MS HBLs and help to predict the performance of graphene-based nanoelectronics and nanocomposites.